Preheat the oven to 300°F.
Place the heavy cream, 6 tablespoons of the demerara sugar and the salt in a small saucepan. Bring the mixture to a low simmer, stirring occasionally, until the sugar dissolves completely.
In a large, deep skillet place the remaining sugar and the water. Bring the mixture to a boil. Heat the sugar mixture until it turns a deep amber, swirling the pan occasionally.
Remove the sugar mixture from the heat and add the warm cream mixture. The sugar mixture will bubble violently. Whisk until everything is well integrated.
In a separate bowl, whisk the egg yolks, calvados, and vanilla together. Take the cream mixture and slowly stream it into the yolk mixture, whisking constantly.
Pour the resulting mixture through a fine mesh strainer into a vessel with a spout. Set aside.
Take four small Mason jars and divide the pot de crème amongst them. Place the jars in a deep 9x13 casserole dish and fill it with enough water to immerse half of each jar. Transfer the casserole dish to the oven and bake for 40 minutes or until the edges are set and the centers have a slight wobble. Remove the jars from the water and let cool to room temperature before transferring to the fridge. Chill the pots de crème for at least an hour and a half before serving.